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Physiology of some Zoophytes. 395 
for receiving the hinge-process; and the smallest is placed at the 
lower end, and affords a passage to the posterior part of the 
pedicle into the interior of the body. The hinge-process is con- 
cave on its upper surface, and terminates below in a curved point. 
Its superior wall forming its concave surface is deficient in two- 
thirds of its length at the upper part or that next the articulation, 
and its inferior or convex wall is very thin over the same extent. 
It is hollow, and communicates with the body through the upper 
and middle apertures seen in its concave or upper surface. Its 
upper or articulating end is bounded by a thickened portion or bar 
passing between the edges of the superior surface, and a similar 
bar passing between the edges of the inferior or convex surface. 
The articulating processes are placed upon the superior of these 
bars, at its junction with the edges of the superior surface. I have 
described, with what may appear very unnecessary minuteness, 
the skeleton of these bird-head processes, because it would be im- 
possible to understand their movements without a previous know- 
ledge of the different parts described. The lateral portions of the 
lower chamber of the body are occupied by two radiating muscles, 
presenting somewhat of the appearance of the temporal muscle in 
the human species, which converge at the articulating or upper 
edge of the hinge-progess, and terminating in a denser, thicker 
and narrower structure, which | shall call tendons, are attached 
to and move this process (fig. 12c). One of these muscles, which 
is the stronger, terminates in a tendon which runs above the 
transverse bar which separates the upper from the middle aperture 
in the concave surface, and running down the centre of the hinge- 
process is inserted into the inner surface of its inferior or convex 
wall a little above its apex or free extremity. When this muscle 
contracts, the hinge-process is tilted up. The other muscular 
bundle, which is strongest at the upper and lower edges, terminates 
in a tendon which passes beneath the bar, and is inserted into 
the hinge-process close to and a little above the tendon of the 
other muscle. When this muscle contracts, the hinge-process, 
if elevated, is drawn down. ‘The first-described muscle is the 
elevator, the second is the depressor muscle of the hinge-process. 
The movements of the hinge-process are in general slight, but I 
have frequently observed it to be tilted up with considerable 
force, and closely applied over the superior surface of the anterior 
chamber, so that its concave, which was before its superior, be- 
came its inferior surface, and its convex became its superior sur- 
face. In this state it may remain for hours, and affords an ex- 
cellent opportunity for observing the arrangement of the fibres 
of the two muscles, especially that of the elevator, as its lower 
fibres run more directly upwards, and its tendon is raised and 
separated from that of the depressor muscle. In dead specimens 
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